The present invention relates to data storage systems.
A data storage system usually includes multiple storage devices, which can be, for example, disk drives (commonly referred to as disks). Unless the context indicates otherwise, the term disk is used in the present specification to embrace not only magnetic storage devices, but also optical, solid state, and other types of storage devices.
The disks of the data storage system can be organized into groups of multiple disks. A RAID array is an example of such a group. (“RAID” is an acronym for Redundant Array of Independent Disks).
During the operation of the data storage system, a disk in a group can fail, i.e., become incapable of correctly executing commands. In a RAID array, data on a disk that has failed must be reconstructed or recovered onto a new disk, which may be swapped into the array or may be a spare already in the array, from other disks in the array. For RAID arrays having single parity data, reconstruction or recovery may not be possible if two disks have failed. For RAID arrays having double parity data, for example, arrays implemented using RAID-DP™ technology available from Network Appliance, Inc. of Sunnyvale, Calif., reconstruction or recovery is generally possible for two disk failures and, depending on configuration and mirroring, even three, four, or five disks failures.
During reconstruction or recovery, a RAID array operates in a degraded state. Because disk failures can occur at inopportune times, for example, during high demand periods, RAID arrays can be forced to operate at a degraded state at these times.